Continuous digester with pressure relief outlet valve cycling

ABSTRACT

Apparatus for the continuous pulping of a compacted mass of wood chips in an upright elongated reaction vessel having an inlet for feeding wood chips substantially continuously into the lower end of the vessel and an outlet for discharging them substantially continuously from the upper end of the vessel, the mass of wood chips being advanced upwardly through the vessel for treatment in liquid submergence by generating liquid propelling pulses by utilizing a top steam outlet having a valve and a cycling timer for operating it, such propelling pulses causing the rate of movement of the submergence liquid relative to the wood chips in the mass to vary cyclically to produce a cyclic advancing movement of the liquid at a rate at least not less than that of the alternating internating movement of the liquid, such advancing movement of the liquid advancing the wood chips throughout the length of the column from the input to the output end thereof.

March 5, 1974 CONTINUOUS DIGESTER WITH PRESSURE RELIEF OUTLETVALVE'CYCLING Original Filed May 29, 1967 STEAM fi% 1-.

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I COUNTER CURRE 50 Q WASHIAII/G ZONE I IHHIHH v I 7 n 53 1, H COOKINGAND -TE HEATING 2L ZONE {MN 4 AP v IMPREKlNAT/ON i 4! 0 AND 5 i} HEATING30 1 I 34 ZOIIVE l i i J wk :2 CHIP COMPACTION United States Patent US.Cl. 162-237 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus for thecontinuous pulping of a compacted mass of wood chips in an uprightelongated reaction vessel having an inlet for feeding wood chipssubstantially continuously into the lower end of the vessel and anoutlet for discharging them substantially continuously from the upperend of the vessel, the mass of wood chips being advanced upwardlythrough the vessel for treatment in liquid submergence by generatingliquid propelling pulses by utilizing a top steam outlet having a valveand a cycling timer for operating it, such propelling pulses causing therate of movement of the submergence liquid relative to the wood chips inthe mass to vary cyclically to produce a cyclic advancing movement ofthe liquid at a rate at least not less than that of the alternatingintervening movement of the liquid, such advancing movement of theliquid advancing the wood chips throughout the length of the column fromthe input to the output end thereof.

This is a continuation of application Ser. No. 132,589, filed Apr. 8,1971 (now abandoned), which was a continuation of application Ser. No.37,435, filed May 14, 1970 (now abandoned), which in turn was acontinuation of application Ser. No. 641,793, filed May 29, 1967 (nowabandoned).

This invention relates to the continuous digesting of wood chips in anupright elongated digester vessel, and more particularly to novelapparatus and methods for continuously advancing a compacted mass ofwood chips throughout the length of such a vessel by generating liquidpropelling pulses through the cycling of a pressure relief outlet valve.

Upfiow digesters of the type shown and described in US. Pat. No.2,878,116 and Pulp and Paper, New Continuous Digester of September 1959have proved to be highly successful because of their thermal stabilityand their unique ability to provide chip drainage, immediately preceded,if desired, by high temperature countercurrent washing, as compared withdownflow systems. A short stroke bottom piston is utilized in thesedigesters to provide the force both for creating and maintaining thelower end of the compacted mass of wood chips and advancing the entiremass upwardly throughout the length of the digester in spite of thepresence of multiple strainers and countercurrent flow and drainagezones and the like.

More recently, as described in copending applications Ser. Nos. 508,531,now abandoned, and 548,225, now abandoned, of common assignor herewith,the compaction of wood chips on the lower end of the'chip mass and theadvancing of the entire mass upwardly through the digester has beenaccomplished by advancing liquid upwardly through the lower portion onlyof the chip mass at a liquid flow rate effective to create a force whichwill propel upwardly the entire chip mass extending throughout thelength of the digester, preferably by providing liquid propelling pulsesas produced by a liquid propelling a ICC pulse generating meanseffective to cause the rate of movement of the submergence liquidrelatively to the wood chips in at least a portion of said mass to varycylically. These liquid propelling pulses provide an average netpropelling pulse froce acting in the submerged chip mass which is highlyeffective to advance the wood chips throughout the length of the woodchip column from the input to the output end thereof, in either upflowor downflow vessels. The liquid flow also may serve to position thelower end of the chip mass, so that the level of compacted chipsdeposited thereon may be kept uniform.

A unique advantage of upflow digesters is their capability of providinga drainage zone at the top, for draining spent liquor from the cookedchips prior to further treatment, in order to increase the efliciency ofchemical recovery. However, the substantial weight of the drained chipsin such a drainage zone opposes the advance of the chip mass lower inthe digester.

Too, in multiple zone digesters, a number of strainers are providedaround the perimeter of the digester at various locations therealong forwithdrawing liquor in order to accomplish heating, treating or washingof the chip mass in the various zones. These also have the adverseeffect of retarding the advance of the compacted chip mass, as chipstend to cling to the strainer surfaces because of the pressure of theliquor being withdrawn.

Furthermore, the countercurrent flow zones used to advantage in suchmultiple zone digesters for carrying out a number of sequential processsteps such as heating and washing produce retarding forces on theadvancing chip mass.

Because of these factors, the forces which must be created in order tooppose the resistance within the vessel must be quite high if the chipmass is to be advanced. For this reason, the short stroke bottom pistonheretofore utilized in commercial upflow digesters has been relativelymassive and hence a major factor in their cost and mechanicalcomplication, while in the case of liquid propulsion in the absence ofpulses, high flow rates and hence large pumps and strainers have beenneeded. In either of these systems, the application of the high forcesapplied at the lower end of the chip mass has tended to compact portionsof the chip mass thereabove to an undue degree which cause undesirablevariations in the pulping recation. Although the propelling pulse systemof application Ser. No. 548,225 is not subject to these disadvantages,the specific pulse generating means thereof requires for feeding chipsinto the digester the use of a constant displacement positive sealingpump of the type shown and described, for example, in Pat. No.2,908,226, which pump is of relatively high cost and is subject to wearproblems not present in much less expensive types of pumps, such ascentrifugal vane pumps, for example. N

Accordingly, it is a major object of the present invention to providenovel methods and apparatus for propelling the compacted mass of woodchips throughout the length of the digester vessel While maintaining amuch more uniform compaction of the chips in the mass than hasheretofore been possible.

In general, such is accomplished by feeding the wood chips into thereaction vessel adjacent its entrance end to create and maintain acolumnar mass of compacted wood chips therein; advancing them throughoutthe length of the mass while treating them in liquid submergence; andthen discharging the treated wood chips from the other end of thecolumn, the wood chip advance being accomplished by liquid propellingpulses. Such pulses are provided by propelling pulse generating means,including pulse timing means and vessel pressure relief valve means,preferably a steam outlet valve connected at the top of the vessel,responsive to the timing means effective to cause the rate of movementof the submergence liquid relatively to the wood chips to varycyclically, providing a Wood chip propelling force eifective to advancethe mass of wood chips continuously to the discharge means. Thesystenrpreferably also includes a propelling liquid recirculatingstrainer in the vessel spaced a substantial disstance from its infeedand a substantial distance from its discharge means for removal ofliquid from the vessel. This establishes a liquid propelling zone withinthe vessel preferably in the lower portion of the wood chip mass belowthe propelling liquid strainer through which propelling liquid means,preferably of the centrifugal vane pump type, is effective to advanceliquid through the propelling zone.

For the purpose of fully explaining the invention, reference is now madeto the following detailed description of a preferred embodiment thereof,together with the accompanying drawing showing a diagramatic sideelevation, partly in section, of apparatus according to the invention.

Referring to the drawing, the pressurized vessel 12 therein shown isgenerally cylindrical except for its lower tapered bottom end portion 13and may be, for example, of the order of ten feet or greater in diameterand one hundred feet in length. Preferably, vessel 12, although ofgenerally uniform cross-sectional area above its tapered bottom endportion 13, has a slight taper to provide a gradually upwardlyincreasing cross-sectional area, a taper of about inch of diameter perfoot of length being suitable. A wood chip-liquid infeed is provided atthe lower entrance end of vessel 12 in the form of a pipe 15 having atangential inlet 16 to which is fed, by means of a motor 17 driving apump 18, preferably of the recessed vane centrifugal type of Pat. No.2,635,548, a wood chip-liquid mixture from a supply tank 20. At theupper end of the vessel 12 is provided an inlet line 14 for supplyingsteam under pressure from any suitable source. There is also provided adischarger for the pulped wood chips in the form of a scraper 22 whichestablishes a predetermined upper level of the wood chip mass bydischarging drained pulped wood chips through discharge outlet 24 to adischarge mechanism (not shown) such as is described in Pat. No.3,206,356.

Within vessel 12, generally along the central vertical axis thereof, areprovided a series of three concentric injection pipes 41, 43, and 53having outlets 42, 44, and 54, respectively, at successively higherlevels. Also within vessel 12 are provided stainers 46 and 48, withtheir valves 51 and 52, respectively, together with said outletsestablishing a plurality of zones within the vessel by advancing liquidfor either co-current or countercurrent flow within said zones asdesired. For example, as shown in the drawing, there is provided alowermost chip compaction zone with a superposed impregnation andheating zone followed by a heating and cooking zone with cocurrent andcountercurrent flow and a countercurrent washing zone with an uppermostdrainage zone, the

liquid flows being shown by the single arrows and the wood chip movementby the double arrows. A liquid level controller 50 is provided near theupper end of vessel 12 to establish the liquid level below scraper 22and the upper level of the wood chip mass as by operating valve 52 ofupper strainer 48 to provide the chip drainage zone.

A chip mass control system is provided for controlling the position ofthe lower end of the chip mass normally spaced above the lower end ofthe vessel for compacting chips thereon continuously to establish andmaintain said lower end at a desired level, in order that the jamming ofwood chips within inlet pipe 15 may be avoided. The apparatus componentsof such system include a lower liquid strainer 30 spaced a substantialdistance preferably of about one-half to one and one-half times thevessel cross-section diameter above inlet pipe 16 and below scraper 22defining a chip compaction zone therebelow and a connecting pipe 34 and38 and valves 69 and 75,

the functions of which are hereinafter described, for controlledrecirculation of liquid through supply tank 20 and pump 18 to inlet 16.

The lower tapered vessel portion 13 of upwardly increasing cmsssectional area is utilized, in conjunction with the upward flow ofpropelling liquid, normally to maintain the lower end L of the columnarmass of wood chips within the tapered vessel portion, suitablypositioned below the propelling liquid strainer 30 and above vesselinlet 16. This is made possible both because of the rapid pressurechange with distance which occurs as a result of resistance to fluidflow within the compacted chip mass, as contrasted to the lack of suchpressure change in the liquid filled column below the chip mass, and thevelocity change with distance which occurs throughout the tapered vesselportion 13, such being greatest at liquid inlet 16 and progressivelydecreasing upwardly therefrom as the vessel cross section increases. Apair of sensors 62, 64 may be provided near the lower end of vessel 12to control the position of the lower end of the chip mass throughcontroller 68 as by operating valve 69 to vary the flow of recirculatingliquid and hence the rate of movement of the wood chip mass.

In accordance with the present invention, liquid pulse generating meansare provided for causing the rate of movement of the submergenceliquidrelatively to the wood chips in the mass to vary cyclically forpropelling the entire columnar chip mass through the vessel.Specifically, such means consists of a top steam outlet line 73 having asteam pressure relief valve 75 operated by a cycle timer 77, so thatreduction of steam pressure at the top of vessel 12 is cyclicallyeffected. Any variations in recirculating liquid when this occurs areabsorbed at supply tank 20, the level of which varies throughout thedistance Ah, but such variations are small enough so that the flow ofliquid and chips through pump 18 to vessel inlet 16 is not significantlyalfected thereby. Nor does the change AH in the upper level in vessel 12seriously affect the liquid flow below strainer 30, although up- Wardfiow of liquid occurs above said strainerduring the portion of the cyclein which valve 75 is closed.

It is a particular feature of the system of the invention that it makespossible the use as wood chip-liquid infeed pump 18 of a centrifugalvortex pump, characterized be a relatively small pressure change over awide range of flow rates, to the extent. that the pump outlet pressureis nearly constant from zero flow up to the maximum flow rate of thepump. In addition, such a pump is uniquely suitable for pumping amixture such as woo chips and water, and is much less costly than thesocalled thick stock pumps heretofore used for the purpose. Conventionalcentrifugal pumps, are also useful with the cycling top pressure systemof the invention.

It has been established in the commercial operation of digesters of thetype described in Pat. No. 2,878,116 that the wood chips are presenttherein in the form of a columnar mass of contacting and compacted butdiscrete wood chips through which liquid can flow more or lessindependently of the wood chips in the mass but subject to a flowresistance due to the presence of the wood chips. Because the wood chipsare present in the mass in discrete form but in contact with oneanother, the wood chips within the mass can be moved throughout thelength of the vessel solely by pushing on the bottom end surface of themass within the vessel regardless of the direction of liquid flow. Suchoperation in fact occurs in digesters of the type described in said Pat.Nos. 2,878,116 and 3,061,007, the latter describing a digester in whichthe top portion of the chip mass extends out of submergence into adrainage zone abovethe surface of the liquid in the vessel, so that theweight of such portion is completely unsupported by the liquid. Withsuch digesters, however, the degree of wood chip compaction tends not tobe uniform throughout the length of the chip column, so that the woodchip digestion is not as uniform as is desired. Furthermore, theforaminous reciprocating piston tends to be unduly massive.

As disclosed in application Ser. No. 548,225, the entire compacted chipmass may be advanced uniformly by subjecting it, preferably throughoutits entire submerged length, to cyclical liquid advancing pulses createdin the sub'mergence liquid. By so doing, non-uniform chip compactionthroughout the length of the chip column is eliminated. Furthermore,since the forces needed at the lower inlet end of the wood chip columnfor carrying out chip accretion and compaction thereon are relativelysmall, such accretion and compaction may be accomplished either by themechanical piston of said patents, but with much less massive apparatus,or with the propulsion liquid approach of application Ser. No. 508,531,but with much lower flow rates and propulsion column lengths. In adownflow system, the action of gravity will serve to form a compactedchip mass.

Furthermore, by applying the principle of tapering the lower portion 13of vessel 12, suitably high fluid flow velocities are readily created inthe tapered liquid column at the inlet end of the vessel so that thefree wood chips are lifted away from the lower end thereof, yet almostany desired low velocity may be created and maintained within thetapered columnar chip mass for propelling it. In addition, the use of atapered lower vessel portion provides self-regulating control of thelower end of the chip mass to an adequately uniform predetermined levelwithin tapered vessel portion 13. This apparently occurs because of thelarge pressure changes which occur within the lower end of the taperedcolumnar wood chip was as a result of increasing liquid velocitiescaused by decreasing vessel cross section area. Thus, as chips acrete onthe bottom of the chip mass, its levels L will move downwardly. Theadded increment will create increasingly higher propelling forces due tothe higher liquid velocities through it as the vessel crosssectiondecreases. This has been found to result, in a fairly short timeinterval, of the entire chip mass moving up- Ward, whereupon the processrepeats itself. Self-regulating level control of the lower end L of thechip mass may be thus achieved.

Both the taper and the length of the propelling zone in relation to thediameter or other transverse dimension of the chip mass is important inmaintaining flow uniformity and in preventing channelling, that is, theopening of large liquid flow channels through the chip mass as tends tooccur between closely spaced fluid inlets and outlets. It is preferredthat the propelling zone be of at least the same order or somewhatgreater than that of the transverse dimension of the chip mass, butcertainly not less than about one-half of said dimension, as with shortpropelling zones, the necessary liquid flow velocity tends to becomeunduly high. It is also preferred that the vessel taper to across-sectional area of at most about one half to one third that of itsmaximum cross-sectional area at the upper end of the propelling zone atpropelling liquid strainer 30.

In general, the use of liquid advancing pulses must be effective tocause the rate of movement of the submergence liquid relatively to thewood chips in the mass to vary cyclically between an advancing movementof the liquid at a rate at least not less, and preferably greater, thatthat of the alternating intervening movement of the submergence liquid.'In fact, as hereinafter more fully explained, it is desirable that suchadvancing liquid pulse flow take place in turbulent flow, with theintervening pulse in laminar flow. However, in considering the precisenature of the pulses, the effect of the vessel configuration on theresistance to movement of the compacted chip mass therein must be takeninto consideration, and the most important of these considerations hasto do with the amount and direction of taper of the vessel. Thus,neglecting the effects of strainers, continuous liquid flows, length ofchip column out of submergence, and the like,

an untapered vessel will have equal resistance to movement of the chipmass therein in either direction, whereas a tapered vessel will have alower resistance to such movement in the direction of increasingcross-sectional area than in the opposite direction. In the case of avessel in which the vessel cross-section increases in the direction ofadvance of the wood chip mass, it does not appear to be essential intheory that the advancing pulse produce a greater speed of liquidmovement than the intervening pulse, although it is desirable that it beat least as great and preferably greater since the amount of vesseltaper is preferably limited in order that a preferred amount of chipcompaction occur, which we have found does not occur if the taper muchexceed about one inch in vessel crosssection per foot of length. In anuntapered vessel, it appears to be essential that the advancing pulseproduce a much greater rate of liquid movement than that provided by theintervening pulse.

The use of an asymmetric pulse has the further advantage of exploitingthe difference between laminar and turbulent liquid flow for producingthe advancing force within the chip mass. This result is made possiblebecause, as is known from hydraulic theory, the rate of fluid flowresistance increases much more rapidly with increased flow velocitywithin the turbulent flow region that it does within the laminar flowregion. Thus, within the region of laminar flow, the resistance to flowis directly proportional to the flow velocity, whereas within theturbulent flow region it increases as the power (n) of the flow, where nis greater than unity. The resistance to flow created by the presence ofthe columnar mass of chips on which the liquid pulse of the presentinvention acts can be determined by measuring the difference (AP)between the fluid pressure (P at the bottom of the chip column and thefluid pressure (P at the top of the chip column. With fluid increasingfrom zero, the resistance (AP) will rise proportionally throughout thelaminar flow region, and then rather abruptly change in a shorttransition region to a much more rapid rise as the flow enters theturbulent region. In this way, the beginning of the turbulent flowregion for a specific chip mass can readily be determined so that afluid flow well within it, that is beyond the transition region, canreadily be selected. Since, as stated in said application Ser. No.508,531, turbulent flow occurs within a compacted chip mass at far lowerflow rates than had heretofore been assumed, liquid propulsion of theentire columnar chip mass can be achieved using pulsed liquid flowwithin the chip mass at flow rates not too high to be provided byconventional means, yet high enough to be within the turbulent flowregion. These flows create suitably high flow resistance within thecolumnar chip mass to move it as desired so that the compacted woodchips can be advanced upwardly throughout the entire length of thecolumnar mass maintained within the vessel. The propelling force socreated is sufficiently high to move the chip mass even if there becountercurrent liquid flow throughout portions of the wood chip mass anda wood chip drainage zone at the upper end of said mass. The ability toprovide such a highly desirable multiple zone upflow system with itsinherent economics in heating, chemical retention and chip drainage is aparticularly important feature of the present invention.

The wood chip advancing force is generally proportional to thedifference between the advancing pulse liquid flow rate (raised to thepower n) and the intervening liquid flow rate (raised to the power n).This difference will be more pronounced if only the advancing pulse bein turbulent flow, where n is greater than about 1.5, usually within therange of about 1.5 to 2.5, as is preferred. Although the actualdifference depends greatly on the porosity of the chip mass and sovaries depending on the chip size, compressibility and the amount ofsawdust or slivers included, the value needed for a particular digestercan be determined from the characteristics and dimensions of the chipmass treatment zones maintained therein, the condition of such zones asto non-submergence or submergence, and the forces created because offactors such as the presence of strainers and fluid flow forcesco-current or countercurrent.

To operate the digester of the drawing according to the presentinvention, it is simply necessary to provide fluid pulses by cyclicallyopening and shutting valve 75 at a frequency of preferably about 1 to 30cycles per minute but at least about 5 to cycles per hour as by valvetimer 77, for example, an open time of about one second and a closedtime of about five seconds to product a rapid height change (AH) in thelevel of submergence liquid in vessel 12 of about one to six inchesfollowed by a slower restoring of its level. These pulses act preferablyin axial liquid upflow throughout the entire submerged portion of thechip mass to cause the rate of movement of the submergence liquidrelatively to the wood chips in said mass to vary cyclically between anadvancing movement of the liquid and an intervening movement of theliquid alternately to raise and lower the upper level of saidsubmergence liquid through the limited distance (AH).

The advancing movement of the liquid through the entire submergedportion of the chip mass is at a rate preferably in turbulent flow,while the intervening movement is in laminar flow, providing a wood chipadvancing movement, when opposed by the resistance within said vessel toadvancing movement of said mass, substantially greater than any woodchip retreating movement produced by the intervening liquid movementthrough said chip mass, when opposed by the resistance within saidvessel to retreating movement of said mass. Such resistance in the caseof the preferred tapered vessel of increasing cross-section in thedirection of wood chip advance is so much greater than it is in theopposite direction of chip mass movement that the chip mass advancesupwardly in stepwise manner without intervening retreating steps.

The position L of the lower end of the compacted chip mass may bepredetermined by varying the recirculation through pipe 34 and supplytank 20. To accomplish this result, the recirculation is increased ifthe lower end of the chip mass moves downwardly and is decreased if thelower end of the chip mass moves upwardly, such being controlled by thetapered bottom portion 13 as well as by sensors 62 and 64 and valve 69.Alternately, the bottom level L of the chip mass may be controlled byvarying the pulse flow rate or the pulse frequency.

Otherwise, the digester operates in a conventional manner, with uncookedchips entering through tangential inlet 16 and being discharged atoutlet 24, the other liquid flows within the digester being atconventional values as may be needed for treatment of the wood chipswithin the vessel.

With pulse propulsion, the advancing pulse of liquid acts on thesubmerged portion of the columnar mass of compacted wood chipsthroughout its entire length. Thus, if vessel resistance tends tocompact one section of the mass to a greater extent than another, thepressure drop through such section will increase. This will increase thepropulsive force in that section which will in turn act to decrease thepacking. Conversely, if a section of the mass becomes loosely packed,the propulsive force will decrease, since if the liquid flows freelythrough loose chips, no propulsive force will be created. As a result, aloose section of the mass will not be advanced until the following chipsin the mass advance to recompact such loose section. If, in actualoperation of the digester of the drawing, any unusual resistance ofmovement of the chip mass is encountered, such as from a partial plug ofchips clinging to a strainer, the amplitude or rate of the advancingpulse can be increased to provide additional liquid movement as neededto free the plug. The invention is also useful in downfiow digesters,since the forces it creates are not limited merely by the force ofgravity acting on the chips.

Various modifications of the invention within the spirit thereof and thescope of the appended claims will be apparent to those skilled in theart.

What is claimed is:

1. Apparatus for the continuous pulping of wood chips in a reactionvessel by feeding substantially continuously said wood chips and aliquid into said reaction vessel adjacent to the entrance end thereof tomaintain a columnar mass of compacted wood chips therein; advancing thewood chips throughout the length of said mass meanwhile treating thesame in liquid submergence, and substantially continuously dischargingsaid wood chips from the exit end of said column, said apparatuscomcomprising an upright elongated reaction vesselhaving an entrance endand an exit end,

wood chip liquid infeed means adjacent the entrance end of said vesselfor feeding wood chips and liquid into the entrance end of said vesseland onto the end of said mass,

wood chip discharge means adjacent the exit end of said vessel, and

liquid propelling pulse generating means including pulse timing meansand vessel pressure relief valve means connected to the top of saidvessel and connected and responsive to said timing means to cause therate of movement of the submergence liquid relatively to said wood chipsto vary cyclically, providing a wood chip propelling force for advancingthe wood chips continuously to said discharge means.

2. Apparatus as claimed in claim 1 wherein said wood chip dischargemeans is adjacent the upper end of said vessel and maintains apredetermined level of the top of said mass below the top of said vesselby discharging wood chips from the upper end of said column, and whereinsaid pressure relief valve means is connected to the top of said vesselabove said predetermined level.

3. Apparatus as claimed in claim 2, wherein said wood chip-liquid infeedmeans includes a centrifugal pump having a small pressure change over awide range of flow rates.

4. Apparatus for the continuous pulping of wood chips in a reactionvessel by feeding substantially continuously said Wood chips and aliquid into said reaction vessel adjacent to the lower entrance endthereof to maintain a columnar mass of compacted wood chips therein;advancing the wood chips upwardly throughout the length of said massmeanwhile treating the same in liquid submergence, and substantiallycontinuously discharging said wood chips from said upper end of saidcolumn, said apparatus comprising an upright elongated reaction vesselhaving a lower entrance end and an upper exit end,

Wood chip liquid infeed means adjacent the lower entrance end of saidvessel for feeding wood chips and liquid into the lower end of saidvessel and onto the lower end of said mass,

wood chip discharge means adjacent the upper end of said vessel formaintaining a predetermined level of the top of said mass below the topof said vessel by discharging Wood chips from the upper end of saidcolumn,

said vessel having a tapered vessel portion of upwardly progressivelyincreasing cross sectional area adjacent the lower endof said vesselwith the lower end of said mass normally maintained within said taperedvessel portion by the upwardly progressively decreasing velocity ofpropelling liquid created within said tapered vessel portion, and

liquid propelling pulse generating means including pulse timing meansand top pressure relief valve means connected to the top of said vesselabove said predetermined level and connected and responsive to saidtiming means to cause the rate of movement of the submergence liquidrelatively to said wood chips to vary cyclically, providing a wood chippropelling force for advancing the wood chips continuously upwardly tosaid discharge means.

5. In apparatus as claimed in claim 4, wherein said tapered vesselportion tapers to a minimum crosssectional area of about half of themaximum crosssectional area of said vessel within said propelling zone.

6. Apparatus for the continuous pulping of wood chips in a reactionvessel by feeding substantially continuously said wood chips and aliquid into said reaction vessel adjacent to the lower entrance endthereof to maintain a columnar mass of compacted wood chips therein;advancing the wood chips upwardly throughout the length of said massmeanwhile treating the same in liquid submergence, and substantiallycontinuously discharging said wood chips from said upper end of saidcolumn, said apparatus comprising an upright elongated reaction vesselhaving a lower entrance end and an uper exit end,

wood chip-liquid infeed means including a centrifugal pump adjacent thelower entrance end of said vessel for feeding wood chips and liquid intothe lower end of said vessel and onto the lower end of said mass, saidpump having a substantially constant outlet pressure at varying flowrates,

wood chip discharge means adjacent the upper end of said vesselmaintaining a predetermined level of the top of said mass below the topof said vessel by discharging wood chips from the upper end of saidcolumn,

propelling liquid strainer means in said vessel spaced a substantialdistance of at least about one-half the cross section dimension of saidvessel above said infeed means and a substantial distance below saiddischarge means for removal of liquid from said vessel establishing aliquid propelling zone within said vessel throughout the entirecross-sectional area thereof in the lower portion of said wood chip massbelow said propelling liquid strainer means, and

liquid propelling pulse generating means including pulse timing meansand top pressure relief valve means connected to the top of said vesselabove said predetermined level and connected and responsive to saidtiming means to cause the rate of movement of the submergence liquidrelatively to said wood chips to vary cyclically, providing a wood chippropelling force advancing the wood chips continuously upwardly to saiddischarge means.

7. Apparatus as claimed in claim 6 wherein said vessel has a lowertapered portion of upwardly progressively increasing cross-sectionalarea tapering at its lower end to a cross-sectional area of at mostabout half of its maximum cross-sectional area within said propellingzone, the lower end of said mass normally being maintained within saidtapered vessel portion by the upwardly progressively decreasing velocityof propelling liquid created within said tapered vessel portion.

References Cited UNITED STATES PATENTS 2,635,548 4/1953 Brawley 103l03 A2,845,347 7/1958 Byrne 162248 X 3,193,444 7/1965 Benjamin l62-237 X3,700,548 10/1972 Rich 16217 ROBERT L. LINDSAY, JR., Primary Examiner A.DANDREA, 1a., Assistant Examiner US. Cl. X.R.

